Some tips to help control bagworms

Friday

Jun 16, 2017 at 11:55 AMJun 16, 2017 at 11:55 AM

I recently noticed a heavy bagworm infestation on some Chinese junipers (Juniperus chinensis) growing on a commercial property in New Bern. The bagworms had been present on that site for a number of years, judging by the amount of dead twigs and foliage. While the bags or sacs are empty at this time of year and represent bagworm activity from the previous year, it’s important to remember that many of the newly hatched larvae will stay on the same plants or groupings of plants. If you see the bags, there’s a strong likelihood that you’re looking at an ongoing infestation.

Bagworms attack arborvitae, Leyland cypress, junipers and a surprising number of other plants, including non-conifers. An effective control program begins in the fall or winter, with removal of as many of the current’s year’s bags as you can find. Use scissors or a sharp knife to carry out this task. Only the female bags will contain eggs, and the male bags will be empty. You might be able to tell which is which by checking for exit holes on the bottom ends of each bag, but who’s got time for that? Things will move along much faster if you just get rid of everything. Female bags may contain 500 to even 1,000 eggs, so this really is an important step in bagworm management.

By early May or even late April the following year, you should start scouting for the small caterpillars that have hatched from last year’s bags. Your previous scouting and removal efforts probably won’t get everything, and additionally, some bagworm larvae will “balloon” in from elsewhere on early spring breezes, riding on fine silk threads. Bt products such as Dipel are safe for the applicator and non-target insects, and work well when the caterpillars (larvae) are very young and small.

By the middle of June, you may or may not see continuing success with Bt products. Azadirachtin, bifenthrin and spinosad materials might be worth trying at this point and beyond. You’ll recognize the developing larvae as bagworms when you see multiple small spindle-shaped bags or sacs comprised of small bits of foliage from the host plant. These bags will be a work in progress throughout the summer, reaching their ultimate size in August.

By sometime in August, this year’s generation of larvae will have matured and molted into the pupal stage, and their bags will be firmly attached to twigs of the host plant with strong silk bands that can actually girdle and damage the wood. At this point, insecticides would be of no value.

Later on in August and into September, the male moths (adults) emerge and look for females to mate with. Mating occurs through the walls of the female bags, as the female adults are flightless and never leave their bags. Eggs are subsequently laid, and the females die.

To summarize, your window of opportunity for bagworm insecticide applications will be fairly brief, beginning sometime in May and ending by early August. For the rest of the year, regular inspection of susceptible plants and removal of any bags you find will be the most effective strategy, and wasteful, ineffective insecticide applications should be avoided.

Tom Glasgow is the Craven County Cooperative Extension director. He can be reached at teglasgo@ncsu.edu.

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